// Copyright (c) 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Cubic algorithm, helper class to TCP cubic.
// For details see http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf.

#ifndef NET_QUIC_CONGESTION_CONTROL_CUBIC_BYTES_H_
#define NET_QUIC_CONGESTION_CONTROL_CUBIC_BYTES_H_

#include <stdint.h>

#include "base/macros.h"
#include "net/base/net_export.h"
#include "net/quic/quic_bandwidth.h"
#include "net/quic/quic_clock.h"
#include "net/quic/quic_connection_stats.h"
#include "net/quic/quic_time.h"

namespace net {

class NET_EXPORT_PRIVATE CubicBytes {
public:
    explicit CubicBytes(const QuicClock* clock);

    void SetNumConnections(int num_connections);

    // Call after a timeout to reset the cubic state.
    void Reset();

    // Compute a new congestion window to use after a loss event.
    // Returns the new congestion window in packets. The new congestion window is
    // a multiplicative decrease of our current window.
    QuicByteCount CongestionWindowAfterPacketLoss(QuicPacketCount current);

    // Compute a new congestion window to use after a received ACK.
    // Returns the new congestion window in bytes. The new congestion window
    // follows a cubic function that depends on the time passed since last packet
    // loss.
    QuicByteCount CongestionWindowAfterAck(QuicByteCount acked_bytes,
        QuicByteCount current,
        QuicTime::Delta delay_min);

    // Call on ack arrival when sender is unable to use the available congestion
    // window. Resets Cubic state during quiescence.
    void OnApplicationLimited();

private:
    static const QuicTime::Delta MaxCubicTimeInterval()
    {
        return QuicTime::Delta::FromMilliseconds(30);
    }

    // Compute the TCP Cubic alpha and beta based on the current number of
    // connections.
    float Alpha() const;
    float Beta() const;

    const QuicClock* clock_;

    // Number of connections to simulate.
    int num_connections_;

    // Time when this cycle started, after last loss event.
    QuicTime epoch_;

    // Time when we updated last_congestion_window.
    QuicTime last_update_time_;

    // Last congestion window used.
    QuicByteCount last_congestion_window_;

    // Max congestion window used just before last loss event.
    // Note: to improve fairness to other streams an additional back off is
    // applied to this value if the new value is below our latest value.
    QuicByteCount last_max_congestion_window_;

    // Number of acked bytes since the cycle started (epoch).
    QuicByteCount acked_bytes_count_;

    // TCP Reno equivalent congestion window in packets.
    QuicByteCount estimated_tcp_congestion_window_;

    // Origin point of cubic function.
    QuicByteCount origin_point_congestion_window_;

    // Time to origin point of cubic function in 2^10 fractions of a second.
    uint32_t time_to_origin_point_;

    // Last congestion window in packets computed by cubic function.
    QuicByteCount last_target_congestion_window_;

    DISALLOW_COPY_AND_ASSIGN(CubicBytes);
};

} // namespace net

#endif // NET_QUIC_CONGESTION_CONTROL_CUBIC_BYTES_H_
